Problems associated with the formation and discharge of electrostatic charge during the manufacture and utilization of insulating materials have been recognized for many years. The problem is particularly acute in the photographic industry. The accumulation of charge on film or paper surfaces leads to the attraction of dust, which can produce physical defects. The discharge of accumulated charge during or after the application of the sensitized emulsion layers can produce irregular fog patterns or static marks in the emulsion. The severity of the static problems has been exacerbated greatly by the increases in sensitivity of new emulsions, increases in coating machine speeds, and increases in post-coating drying efficiency. The charge generated during the coating process results primarily from the tendency of the webs of high dielectric constant polymeric film base to charge during winding and unwinding operations (unwinding static), during transport through the coating machines (transport static), and during post-coating operations such as slitting and spooling. Static charge can also be generated during the use of the finished photographic product. In an automatic camera, the winding of film out of and back into the film cassette, especially in a low relative humidity environment, can result in static charging. Similarly, high speed automated film processing can result in static charge generation. Sheet films, such as X-ray films, are especially subject to static charging during removal from light-tight packaging.
It is generally known that electrostatic charge can be dissipated effectively by incorporating one or more electrically conductive antistatic layers into the film structure. Antistatic layers can be applied to one or both sides of the film base as subbing layers either beneath or on the side opposite the light-sensitive silver halide emulsion layers. An antistatic layer can alternatively be applied as an outer coated layer either over the emulsion layers or on the side of the film base opposite the emulsion layers or both. For some applications, the antistatic agent can be incorporated into the emulsion layers. Alternatively, the antistatic agent can be directly incorporated into the film base itself.
A wide variety of electrically conductive materials can be incorporated into antistatic layers to produce a wide range of conductivities. Most antistatic layers traditionally used for photographic applications employ ionic conductors. Electric charge is transferred in ionic conductors by the bulk diffusion of charged species through an electrolyte. Antistatic layers containing electronic conductors also have been described. Because the conductivity of electronic conductors depends predominantly on electronic mobility rather than ionic mobility, their observed electrical conductivity is independent of relative humidity and only slightly influenced by ambient temperature. Antistatic layers containing various conjugated polymers, conductive carbon particles or semiconductive inorganic particles have also been described.
Of the various types of electronic conductors, electrically conducting metal-containing particles, such as semiconducting metal oxides, are particularly effective when dispersed in suitable polymeric film-forming binders in combination with polymeric non-film-forming particles as described in U.S. Pat. Nos. 5,340,676, 5,466,567, 5,700,623. Binary metal oxides doped with appropriate donor heteroatoms or containing oxygen deficiencies have been disclosed in the prior art to be useful in antistatic layers for photographic elements, for example, U.S. Pat. Nos. 4,275,103, 4,416,963, 4,495,276, 4,394,441, 4,418,141, 4,431,764, 4,495,276, 4,571,361, 4,999,276, 5,122,445, 5,294, 525, 5,382,494, 5,459,021, 5,484,694 and others. Suitable disclosed conductive metal oxides include zinc oxide, titania, tin oxide, alumina, indium oxide, silica, magnesia, zirconia, barium oxide, molybdenum trioxide, tungsten trioxide, and vanadium pentoxide. Preferred doped conductive metal oxide granular particles include antimony-doped tin oxide, fluorine-doped tin oxide, aluminum-doped zinc oxide, and niobium-doped titania. In addition, conductive ternary metal oxides have been disclosed in U.S. Pat. No. 5,368,995, which include zinc antimonate and indium antimonate. Other conductive metal-containing granular particles including metal borides, carbides, nitrides and suicides have been disclosed in Japanese Kokai No. JP 04-055,492.
One deficiency of such granular electronic conductor materials is that, especially in the case of semiconductive metal-containing particles, the particles usually are highly colored which render them unsuitable for use in coated layers on many photographic supports, particularly at high dry weight coverage.
The use of “fibrous” or “fibrilar” conductive materials in imaging elements has been taught or disclosed in the prior art. A conductive backing or subbing layer for silver halide photographic films prepared by coating an aqueous dispersion of a colloidal gel of “amorphous” vanadium pentoxide, preferably silver-doped vanadium-pentoxide, onto a film support is described in U.S. Pat. Nos. 4,203,769 and 5,439,785. Colloidal vanadium pentoxide gel consists of entangled conductive microscopic fibrils or ribbons that are 0.005–0.01 μm wide, about 0.001 μm thick, and 0.1–1 μm in length. Conductive layers containing colloidal vanadium pentoxide exhibit low surface resistivities at very low dry weight coverages, low optical losses, and excellent adhesion to the support. However, since colloidal vanadium pentoxide dissolves in developer solution during wet processing of photographic products, it must be protected by a nonpermeable, hydrophobic overlying barrier layer as taught in U.S. Pat. Nos. 5,006,451, 5,284,714, and 5,366,855.
Composite conductive particles consisting of a thin layer of conductive metal-containing particles deposited onto the surface of non-conducting transparent core particles are known in the art. These composite particles provide a lightly colored material with sufficient conductivity. For example, composite conductive particles consisting of two dimensional networks of fine antimony-doped tin oxide crystallites in association with amorphous silica deposited on the surface of much larger, non-conducting metal oxide particles (e.g., silica, titania, etc.) and a method for their preparation are disclosed in U.S. Pat. Nos. 5,350,448, 5,585,037 and 5, 628,932. Alternatively, metal containing conductive materials, including composite conducting particles, with high aspect ratio can be used to obtain conductive layers with lighter color due to reduced dry weight coverage as described, for example, in U.S. Pat. Nos. 4,880,703 and 5,273,822. However, there is difficulty in the preparation of conductive layers containing composite conductive particles, especially composite particles having a high aspect ratio, since the dispersion of these particles in any vehicle using conventional wet milling dispersion techniques and traditional steel or ceramic milling media often results in wear or abrasion of the thin conducting layer from the core particle and/or reduction of the aspect ratio. Fragile composite conductive particles often cannot be dispersed effectively because of limitations on milling intensity and duration dictated by the need to minimize degradation of the morphology and electrical properties as well as the introduction of attrition-related contamination from the dispersion process.
Electrically conducting polymers have recently received attention from various industries because of their electronic conductivity. Since the discovery in 1977 that polyacetylene, when doped, has conductivity 109 times greater than the undoped polymer, many new materials have been synthesized and commercialized. The commercialization exemplified by the following list of materials illustrates the growth in the area of conductive polymers.
Doped polyaniline is used as a conductor and for electromagnetic shielding of electronic circuits. Polyaniline is also manufactured as a corrosion inhibitor. Poly(ethylenedioxythiophene)(PEDOT) doped with polystyrenesulfonic acid is manufactured as an antistatic coating material to prevent electrical discharge exposure on photographic emulsions and also serves as a hole-injecting electrode material in polymer light-emitting devices. Poly(phenylenevinylene) derivatives have been major candidates for the active layer in the manufacture of electroluminescent displays for use in applications such as mobile telephone displays. Poly(dialkylfluorene) derivatives are used as the emissive layer in full-color video matrix displays. Poly(thiophene) derivatives are promising field-effect transistors. Poly(pyrrole) has been tested as a microwave-absorbing “stealth” (radar-invisible) screen coating and also as the active thin layer of various sensing devices. Electronically conductive polymers show very high conductivity when doped. Suitable dopants include halogens such as iodine, chlorine, and bromine. Other suitable dopants include inorganic mineral acids such as hydrochloric acid and various strong organic acids such as methanesulfonic acid, camphorsulfonic acid, and styrenesulfonic acid.
Although many electronically conductive polymers are highly colored and are less suited for photographic applications, some of these electrically conducting polymers, such as substituted or unsubstituted pyrrole-containing polymers, as mentioned in U.S. Pat. Nos. 5,665,498 and 5,674,654, substituted or unsubstituted thiophene-containing polymers as mentioned in U.S. Pat. Nos. 5,300,575, 5,312,681, 5,354,613, 5,370,981, 5,372,924, 5,391,472, 5,403,467, 5,443,944, 5,575,898, 4,987,042 and 4,731,408, and substituted or unsubstituted aniline-containing polymers as mentioned in U.S. Pat. Nos. 5,716,550, 5,093,439 and 4,070,189 are transparent and not prohibitively colored, at least when coated in thin layers at moderate coverage. Because of their electronic conductivity instead of ionic conductivity, these polymers are conducting even at low humidity. It has been observed in the industry that loss of electrical conductivity after wet processing may increase dirt attraction to processed films which, when printed, may cause undesirable defects on the prints. The present polymers can retain sufficient conductivity even after wet chemical processing to provide what is known in the art as “process-surviving” antistatic characteristics to the photographic support to which they are applied. Unlike metal-containing semiconducting particulate antistatic materials (e.g., antimony-doped tin oxide), the aforementioned electrically conducting polymers are less abrasive, and environmentally more acceptable (due to absence of heavy metals). However, it has been reported in U.S. Pat. No. 5,354,613 that the mechanical strength of a thiophene-containing polymer layer is not sufficient and can be easily damaged.
Electrically conductive layers are also commonly used in imaging elements for purposes other than providing static protection. Thus, for example, in electrostatographic imaging it is well known to utilize imaging elements including a support, an electrically-conductive layer that serves as an electrode, and a photoconductive layer that serves as the image-forming layer. Electrically conductive agents utilized as antistatic agents in photographic silver halide imaging elements are often also useful in the electrode layer of electrostatographic imaging elements.
Besides imaging elements, other display products and accessories, particularly those involving electronic display, may require electrical conductivity. Glass and plastics molded parts become electrostatically charged by friction or application of charges, for example electron beams in TV picture tubes. As a result of these charges the parts rapidly become covered with dust due to attraction of dust, which is undesirable in practice. To achieve protection, the parts can be coated with an antistatic coating. In addition, a screening effect against electromagnetic radiation, as is emitted for example from cathode ray tubes, is also achieved with sufficiently conducting materials. High conductivity may also be required for certain accessories in electronic display products, in order to be part of the electrical circuitry.
For practical use these coatings must also have a sufficient mechanical strength and adhesion. Especially in the case of glass as a carrier, the layers must be sufficiently durable in order to avoid damage to the coating during cleaning or other usage of the coated surfaces resulting in the loss of the antistatic and/or conducting effect. Electrically conducting polymers, for example polythiophenes, for producing antistatic and/or conducting coatings on glass are known from the literature. However, it has been noted in U.S. Pat. Nos. 6,201,051 and 6,004,483, that these coatings are not sufficiently scratch-resistant in practice for some applications.
As indicated above, the prior art on electrically-conductive layers in imaging elements is extensive and a very wide variety of different materials have been proposed for use as the electrically-conductive agent. There is still, however, a critical need in the art for improved electrically-conductive layers which are useful in a wide variety of imaging elements, which can be manufactured at reasonable cost, which are environmentally benign, which are durable and abrasion-resistant, which are effective at low coverage, which are adaptable to use with transparent imaging elements, which do not exhibit adverse sensitometric or photographic effects, and which maintain electrical conductivity even after coming in contact with processing. Thus, for a wide variety of applications there is a critical need for conductive particles; which are electronically conductive, transparent, durable, dispersible for easy formulation and which have a high aspect ratio in order to provide necessary conductivity at a low coverage without any objectionable color.